Digital watermarking refers to the embedding of a hidden message in an image or image sequence for such purposes as establishing ownership, tracking the origin of the data, preventing unauthorized copying, or conveying additional information (meta-data) about the content. Watermarking has potential uses in a wide range of products, including digital still and video cameras, printers and other hardcopy output devices, and content delivery services (e.g., Internet-based photofinishing). Recently, there has been significant interest in the electronic distribution and display of theatrical movies, which is termed digital cinema. Studios and distributors have a strong need to protect the movie content from unauthorized use, and watermarking can assist by establishing ownership and tracing the source of stolen content (through the use of hidden date/time/location stamps inserted at the time of the movie distribution and/or presentation). The present invention relates specifically to the watermarking of image sequences, and thus it has usefulness in an application such as digital cinema.
Numerous watermarking methods have been described in the prior art, including both patents and the technical literature. Many of these methods are described in review papers such as: Hartung and Kutter, Multimedia Watermarking Techniques,” Proc. IEEE, 87(7), pp. 1079-1107 (1999), and Wolfgang et al., Perceptual Watermarks for Digital Images and Video, Proc. IEEE, 87(7), pp. 1108-1126 (1999).
A basic distinction between various methods is whether the watermark is applied in the spatial domain or the frequency domain. In either approach, it is common for a pseudo-random (PN) sequence to be used in the watermark generation and extraction processes. The PN sequence serves as a carrier signal, which is modulated by the original message data, resulting in dispersed message data (i.e. the watermark) that is distributed across a number of pixels in the image. A secret key (i.e. seed value) is commonly used in generating the PN sequence, and knowledge of the key is required to extract the watermark and the associated original message data.
As noted in the review papers by Hartung et al. and by Wolfgang et al., most research on watermarking techniques has focused on single-frame images, and there are significantly fewer methods that are specific to image sequences (i.e. video watermarking). Of course, a watermarking method that has been designed for single-frame images could be applied to an image sequence by merely repeating the same process for each frame. However, this approach has the disadvantage that the fixed watermark pattern may become perceptually objectionable when the image sequence is displayed in real-time.
There are several prior art patents that include video-specific watermarking methods: U.S. Pat. No. 5,809,139 issued Sep. 15, 1998 to Girod et al. entitled Watermarking Method and Apparatus for Compressed Digital Video; U.S. Pat. No. 5,901,178 issued May 4, 1999 to Lee et al. entitled Post-Compression Hidden Data Transport for Video; U.S. Pat. No. 5,991,426, issued Nov. 23, 1999 to Cox et al. entitled Field-Based Watermark Insertion and Detection; U.S. Pat. No. 6,026,193 issued Feb. 15, 2000 to Rhoads entitled Video Steganography.
In the patents by Girod et al. and Lee et al., the methods are designed for directly embedding a watermark in compressed frequency-domain video streams (such as MPEG-encoded sequences). The patent by Cox et al. describes a method for alternately embedding positive and negative watermarks in consecutive fields of an interlaced video signal; this method is not suitable for progressively scanned image sequences such as those used in digital cinema applications. The patent by Rhoads discloses the basic concept of using multiple watermarked frames from an image sequence to extract the watermark with a higher degree of confidence than would be obtained with only a single frame. However, the methods described in all of the aforementioned patents make use of the same watermarking pattern in each successive frame of the sequence. As a result, these methods are subject to the same disadvantage as previously mentioned, namely, the presence of a fixed watermark pattern that can be objectionable.
There are obvious modifications that can eliminate the fixed watermark pattern, but they also suffer from limitations. One modification is to change the PN carrier from frame to frame, but this may necessitate a brute-force search of all possible carriers when performing the watermark extraction process. The management of the secret keys that are used in generating the PN sequences also becomes problematic. Another modification is to change the message while using the same carrier, but it may not be desirable to change the message from frame to frame in many applications. Moreover, either modification does not allow information from multiple frames to be directly combined when extracting the watermark. This limitation reduces the robustness of the watermark extraction process to certain types of removal attacks.
There is a need therefore to have an image sequence watermarking technique that: (1) minimizes the visibility of the watermark when the watermarked sequence is displayed in real-time, (2) requires only a single key for the generation and extraction of the watermark data, and (3) allows for information from multiple frames to be combined when extracting the watermark.